VLSI circuits such as DRAMs (dynamic random access memories) often have long conductive tracks, such as wordlines extending long distances e.g. 1 cm across a chip. The long conductive tracks contain distributed parasitic elements, distributed series resistance and distributed parallel capacitance. This causes delay and degradation of pulse signals carried along the tracks, so that it takes significantly longer for elements controlled by the pulse signals to be enabled at the end of the track than elements at the beginning of the track. In order to facilitate synchronous operation of the controlled elements, operation of the entire chip is slowed to the speed of the slowest operating elements, i.e. those at the ends of the tracks.
To improve operation of long tracks, in prior art SRAMs (static random access memories) the tracks are divided into two or more segments. Signal refreshing circuits known as repeaters are connected between the end of one segment and the beginning of the next, in order to allow signal flow from one end to the other of the track. The result is a restandardization of the pulse signal passing down the track, compensating for the square law degradation relationship between delay and line length. Unfortunately, while such repeaters have been able to improve the rise and fall time characteristics of a pulse passing down the track, optimizing the delay to rising edges of signals degrades the falling edge delay and vice-versa.
The repeater circuit used in SRAMs could not be used to repeat DRAM wordline signals because DRAM wordline signals are required to be driven to a level above the main supply voltage. In prior art DRAMs this was done using multiphase bootstrap circuits unsuitable for use in repeaters. In SRAM circuit the repeaters could be simple inverters fed from the main supply voltage and therefore could not drive the wordline to the level required for use in a DRAM.
I have invented, as described in U.S. patent application Ser. No. 680,994, now U.S. Pat. No. 5,267,201, a static DRAM wordline driver operated from a high voltage pump supply. While not related to this invention, I have discovered that a repeater as described herein can for the first time be used to improve the pulse characteristics of DRAM wordlines, while previously repeaters could only be used for such purposes in SRAMs.